Non-electroactive ions

The participation of cations in redox reactions of metal hexacyanoferrates provides a unique opportunity for the development of chemical sensors for non-electroactive ions. The development of sensors for thallium (Tl+) [15], cesium (Cs+) [34], and potassium (K+) [35, 36] pioneered analytical applications of metal hexacyanoferrates (Table 13.1). Later, a number of cationic analytes were enlarged, including ammonium (NH4+) [37], rubidium (Rb+) [38], and even other mono- and divalent cations [39], In most cases the electrochemical techniques used were potentiometry and amperometry either under constant potential or in cyclic voltammetric regime. More recently, sensors for silver [29] and arsenite [40] on the basis of transition metal hexacyanoferrates were proposed. An apparent list of sensors for non-electroactive ions is presented in Table 13.1. 

The oxidation reaction replaces Fe ions with Fe " ions in the electrolyte. As far as electroactive species are concerned, it therefore produces an excess of positive charge in the diffusion layer. This charge excess must be compensated by non-electroactive ions. Here, both types of non-electroactive ions of the supporting electrolyte help compensate for this positive charge excess. There is an excess of NOj" anions and a deficit of IC cations in the diffusion layer, when compared to the original concentrations (see figure 4.16). 

In this figure, the concentration scale is proportionately eight times smaller than that In flgure4.15. Therefore the origin (zero concentration) cannot be seen here. The difference between the bulk concentrations of the two non-electroactive ions, [IC] and [NO3T, is explained by the fact that the nitrate Ion Is also the counter-Ion of the two electroactive cations. 

Definition of a Non-electroactive ions in much higher concentrations than those of electroactive ions... 

Kordorouba, V. and Pelletier, M., Ion chromatography using an electrochemical detector response to non-electroactive anions, /. Liq. Chromatogr., 11, 2271, 1988. 

In conclusion, the unique properties of Prussian blue and other transition metal hexa-cyanoferrates, which are advantageous over existing materials concerning their analytical applications, should be mentioned. First, metal hexacyanoferrates provide the possibility to develop amperometric sensors for non-electroactive cations. In contrast to common smart materials , the sensitivity and selectivity of metal hexacyanoferrates to such ions is provided by thermodynamic background non-electroactive cations are entrapped in the films for charge compensation upon redox reactions. 

The speciation of trace metals can be studied analytically using various fractionation schemes [ 2—5 ]. Metal species in solution can be arbitrarily classified by DPASV, as free ions and as both labile and nondabile conqdexed metal species. The species which are classified is nonJabile are non-electroactive under the experimental conditions... 

Deposition of a non-electroactive film on the surface of an electrode blocks the electron transfer from solution-based ions to the electrode. The efficiency of such blocking depends on the permeability of the film and the nature and density of defects, and heterogeneous electron transfer is routinely used to address these problems366. Capacitance measurements of the blocked electrodes also give valuable information about the thickness and integrity of the monolayer. These applications are described in Section II.D. 

The detection of the current generated by reaction at the surface of (usually) carbon fiber or copper microelectrodes at a fixed voltage is capable of low detection limits for electroactive compounds using amperometry, Table 8.14. Several approaches that allow the full possibilities of multiple electrode and pulsed amperometric detection (established techniques in liquid chromatography (section 5.7.4)) have been proven for capillary electrophoresis [508,511]. These methods are not widely used, possibly due to a lack of commercial products and support. Potentiometric detection with polymer-coated wire microelectrodes containing relatively non-specific ion exchange ionophores was used for the detection of low-mass anions or cations [510,511]. 

As in the case of all non-electroactive species, the ions of the supporting electrolyte have components for migration and diffusion movement that are opposite in directions, and compensate each other perfectly in the area next to the electrode . ... 

Functionalized ICP chains can be also used for the detection of non-electroactive alkaline and alkaline earth metals by exploiting quite a different approach the coordination of the ICP molecules with the metal ions affects the physico-chemical properties of the polymer film, inducing a strong variation of the relevant voltammetric trace. In particular, the p-doping process results conditioned by the formation of more or less strong chemical interactions between the metal analyte... 

This section deals with the fabrication of potentiometric probes and their use in SECM studies. Potentiometric probes (see Chapter 7) can detect many non-electroactive species not accessible to amperometric techniques. They are highly selective and have found widespread application in clinical chemistry, in environmental studies and the food industry. A general review of potentiometric probe fabrication has been presented previously, and several publications have demonstrated the utility of potentiometric probes in SECM studies (55). This section will provide the reader with a highlight of potentiometric probe fabrication techniques taken from the literature. The section will also include a discussion of the basic concepts, fabrication steps, necessary equipment, and characterization of ion-selective micropipettes applied in SECM studies. 

The standard electrode potential (q.v.) is often not greatly different in non-aqueous solvents from that in water, although displacements due to differences in the strength of solvation of the ions are to be expected. The same reference electrodes as are used in water are also usually satisfactory. The rates of electrochemical reactions, however, can be radically altered by changes of solvent, since all the factors which govern the ease of transfer of electrons across the electrode surface are likely to be modified. These include the solvation of the electroactive ions, their tendency to ion-pairing and complex formation, the adsorbability of the solvent and of active species at the electrode surface, and the other factors that may affect the structure of the electrical double layer (q.v.). 

Although hydronium ion (H30+) (Chapter 8) and dioxygen (02) (Chapter 9) are the most studied of the molecules and ions without metal atoms, several of the molecules that contain sulfur, nitrogen, or carbon also are electroactive. The results for representative examples are presented to illustrate the utility of electrochemical measurements for die evaluation of the redox thermodynamics and bond energies for non-metal-containing molecules. In particular, die electrochemistry for several sulfur compounds [S8, S02, HS(CH2)3SH], nitrogen compounds [-NO, HON=0, N20, H2NOH, hydrazines (/ NHNH/ ), amines, phenazine], and carbon compounds (C02, CO, NCT) is summarized and interpreted. 

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